Under certain operating conditions a phenomenon referred to a “voltage droop” may lead to an integrated circuit's power supply falling below proper operating levels. A voltage droop refers to a loss of supply voltage as a device tries to drive a load. For example, when power is supplied to a microprocessor, the sudden increase in current through the microprocessor may cause a loss in the supply voltage due to the package inductance. In other words, the sudden increase in the current drawn by the microprocessor causes a voltage droop because there is a high rate of change of current through the microprocessor package inductance.
Voltage droop may occur during heavy loading conditions. In this case, the amount of current demanded by the processor exceeds that which can be provided by the power supply and, as result, the power supply voltage at the transistor level droops. Voltage droop may also occur under conditions where the processor draws current at a certain frequency. Depending on the power variation frequency, the processor may hit the resonance frequency associated with the combination of the package inductance and the chip capacitance. In this case, a voltage droop can occur at the voltage supply at the transistor level. In order to prevent an integrated circuit from operating outside of the appropriate range of power supply voltages, it may be desirable to monitor the power supply voltage and to take steps to mitigate voltage droop.